1. Field of the Invention
The present invention relates to a method and apparatus for converting a wide band audio signal into a series of discrete coded pulses and more particularly a method and means for converting such a signal into coded pulses with a result of having reduced noise and distortion errors.
The invention also relates to any analog-to-digital conversion application where maintaining inherent device accuracy is desired.
2. Description of the Prior Art
It has been demonstrated that instead of transmitting a continuous version of a signal containing intelligence, it is sufficient to sample the signal at regular discrete time intervals and to transmit the samples of the modulating signal in the form of a periodic pulse train which can be subsequently demodulated after transmission via a chosen medium.
Various pulse modulation methods have been used for years on voice grade and telex circuits because of several advantages; which include relative noise immunity, ease of multiplexing by interlacing pulses from two or more signal sources and the ease with which pulses can be reshaped with regenerative repeaters on long transmission circuits. In fact, pulse modulators/demodulators (MODEMS) are supplied to the wire services.
The requirement that the sampling frequency be at least twice the highest frequency in the signal source has limited application of pulse modulators to voice grade communications, teletype and data circuits whose upper frequency limit is about 4 kilohertz.
Digitizing high-fidelity audio material requires a greater degree of accuracy and resolution (14 to 16 bits) and also higher digitizing speed (less than 20 .mu.sec per sample) in order to convert both channels of stereo with an upper cut off frequency of 20 kilohertz. 16 Bit analog-to-digital converters have become available in recent years, but they are very difficult to incorporate into a system in order to take full advantage of their inherent accuracy. In particular prior ADC's have been sensitive to impedance in the analog ground wiring from the ADC to a system ground point. This happens because voltage pulses are generated during the digitizing process due to fluctuating analog ground currents which flow through the wiring impedance. Since digitizing is a high-speed dynamic process, the presence of voltage pulses at the analog ground terminal of an ADC prevents the digitizing process from converging to the desired value. Further, when parasitic inductance exists in analog ground wiring, ringing in the voltage pulses prevents the digitizing process from converging to the same result when repeatedly digitizing the same input voltage. This lack of repeatability manifests itself as noise, an undesirable characteristic in any high resolution application.
It has been observed that wiring impedance in the analog ground can give rise to one or more of the following errors: noise or alternations (non-repeatability), increased linearity and differential linearity errors, shifts in offset and gain errors, missing output codes, hysteresis, and non-monotonicity. For a discussion of these errors in the literature, see "Testing Digital/Analog and Analog/Digital Converters" by Jim Naylor in IEEE Transactions on Circuits and Systems, July 1978. In high-quality PCM digital audio systems these errors cause increased distortion and noise.
Thus a need existed for an ADC which circumvents much of the wiring and layout restrictions that have been imposed upon the prior art.